1. Field of the Invention
The present invention relates to a catadioptric projection apparatus, for example used in fabricating semiconductor devices or liquid crystal display devices, etc. by the photolithography process, which is suitable as a projection optical system for 1:1 or reduction projection. More particularly, the invention relates to a catadioptric projection apparatus particularly using a reflection system as an element in an optical system, thereby attaining a resolving power of the submicron order in the ultraviolet wavelength region.
2. Related Background Art
The projection exposure apparatus is used in fabricating semiconductor devices or liquid crystal display devices by the photolithography process. The projection exposure apparatus is so arranged to demagnify a pattern formed on a photo-mask or reticle (hereinafter generally referred to as "reticle") through a projection optical system for example at a ratio of about 5:1 to transfer a demagnified image of the pattern onto a substrate (wafer, glass plate or the like) coated with a photosensitive material (photoresist or the like). With an increase in degree of integration for semiconductor elements, demands become increasing for a projection optical system with a higher resolving power for use in the projection exposure apparatus.
To satisfy such demands, it is necessary to shorten the wavelength of illumination light or to increase the numerical aperture (NA) of the projection optical system. However, as the wavelength of illumination light becomes shorter, influence of light absorption increases so as to limit the types of glass materials which can be practically used. Particularly in the wavelength range of illumination light below 300 nm, practically usable glass materials are only synthetic quartz, and fluorite.
In this case, the two materials have respective Abbe's numbers which are too close to each other to correct the chromatic aberration. It is thus very difficult for the projection optical system composed only of a refracting system to correct the chromatic aberration. Also, fluorite is poor in characteristics of change of refractive index with a temperature change, i.e., poor in the so-called temperature characteristics. Further, fluorite has various problems in machining of lens polishing. Therefore, fluorite cannot be used for many parts in the projection optical system. Consequently, it is very difficult to realize a higher resolving power by the projection optical system composed only of the refracting optical system.
On the other hand, an attempt has been made to construct a projection optical system only of a reflection optical system, because the reflection optical system causes no chromatic aberration. In this case, it is necessary that the scale of the projection optical system is increased and that the reflective surface is aspherical. Accordingly, it is also very difficult to construct the projection optical system only of the reflection optical system.
Then, there are various techniques proposed to construct projection apparatus for reduction projection of a combination optical system of a reflecting system and a refracting system, i.e., of a so-called catadioptric optical system. Such a catadioptric optical system employs a beam splitter for change of optical path in order to guide a light beam into or out of the reflecting system.
The beam splitter is shaped in a cube or a truncated pyramid. Thus, an optical path length for light first entering the beam splitter to travel therein is equal or nearly equal to that for light reflected by the reflecting system and again entering the beam splitter to travel therein.
The catadioptric projection apparatus including the cubic beam splitter is described in detail for example in Japanese Laid-open Patent Application No. 4-235516 and Japanese Laid-open Patent Application No. 5-72478. Further, the catadioptric optical system including the beam splitter of truncated pyramid is described in detail for example in Japanese Laid-open Patent Application No. 2-66510.